Abstract
Callose is an important biopolymer of β-1,3-linked glucose units involved in different phases of plant development, reproduction and response to external stimuli. It is synthesized by glycosyltransferases (GTs) known as callose synthases (CalS) belonging to family 48 in the Carbohydrate-Active enZymes (CAZymes) database. These GTs are anchored to the plasma membrane via transmembrane domains. Several genes encoding CalS have been characterized in higher plants with 12 reported in the model organism Arabidopsis thaliana. Recently, the de novo transcriptome of a fibre-producing clone of stinging nettle (Urtica dioica L.) was published and here it is mined for CalS genes with the aim of identifying members differentially expressed in the core and cortical tissues of the stem. The goal is to understand whether specific CalS genes are associated with distinct developmental stages of the stem internodes (elongation, thickening). Nine genes, eight of which encoding full-length CalS, are identified in stinging nettle. The phylogenetic analysis with CalS proteins from other fibre crops, namely textile hemp and flax, reveals grouping into 6 clades. The expression profiles in nettle tissues (roots, leaves, stem internodes sampled at different heights) reveal differences that are most noteworthy in roots vs. leaves. Two CalS are differentially expressed in the internodes sampled at the top and middle of the stem. Implications of their role in nettle stem tissue development are discussed.
Highlights
The plant cell wall is a natural composite material composed of cellulose, hemicelluloses, pectins, as well as structural proteins and it can be impregnated by the aromatic macromolecule lignin, which confers hydrophobicity [1]
This study identifies at least nine callose synthases (CalS) genes in stinging nettle and bioinformatics searches have found putative conserved motifs in their promoters recognized by transcription factors (TFs), in particular of the Dof family
From the results obtained with qPCR, the CalS genes identified are all detected in the different organs and stem tissues of stinging nettle and three are differentially expressed along the stem
Summary
The plant cell wall is a natural composite material composed of cellulose, hemicelluloses, pectins, as well as structural proteins and it can be impregnated by the aromatic macromolecule lignin, which confers hydrophobicity [1]. The different composition and mechanical properties of plant cell walls are well exemplified in the stem of higher plants, where a basipetal lignification gradient accompanies the transition from elongation in young tissues at the top, to thickening in older internodes at the bottom [5,6] Besides these classes of biopolymers, plant cells synthesize another type of polysaccharide, callose, amorphous and characterized by β-1,3-linked glucose units with some β-1,6 branches [7]. Callose is deposited in response to pathogen attack [8] or wounding [9], as well as at the cell and sieve plates; it regulates symplastic connectivity through plasmodesmata, by determining the size exclusion limit (SEL) [10] It is present in the inner cell wall of pollen tubes where it resists tension and compression stress [11,12] and it was shown to participate in cotton fibre growth [13]. This last aspect is in relation to symplasmic isolation via plasmodesmata gating ensuring the turgor-driven elongation of the fibre cell [14]
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